This invention is directed to an improved CRT (cathode ray tube) stabilization network for use in color television receivers.
Generally, color television receivers include a CRT which has three electron guns, each of which includes its own cathode. The cathode current of each gun is directed toward the screen of the CRT to develop the red, blue or green component of a video image.
To produce an image of the proper color, the cut-off points of the cathodes should track with one another. This is, a video signal which causes one of the guns to produce a black level current should cause corresponding black level currents to be developed in the other two electron guns. Similarly, a video signal which causes one electron gun to develop a peak white current should cause corresponding peak white currents to be developed in the other two guns.
Because of differences in their temperatures, their aging characteristics and other variables, the emission characteristics of cathodes tend not to track with one another. That is, a video signal which produces a given cathode current in one gun does not produce a corresponding cathode current in the other two guns. As a result, mis-coloration of the video image occurs unless precautions are taken to avoid such mis-tracking.
To overcome the problem of mistracking among cathodes, many television receivers include six manually adjustable potentiometers. Three of the potentiometers are used to adjust the bias which each video amplifier applies to the cathode of an electron gun and three more potentiometers adjust the gain of three video output amplifiers. Typically, these are factory and service adjustments which compensate for the different emission characteristics of the electron guns. Changes in the emission characteristics or improper setting of the potentiometers results in mis-coloration in the video image.
To reduce the number of potentiometers required for cathode tracking, U.S. Pat. No. 3,955,212 teaches that the D.C. output voltage and the gain of each video output amplifier can be simultaneously adjusted by using one potentiometer per output amplifier. Adjusting the D.C. output voltage of each video output amplifier sets the cut-off point of its associated cathode for proper black level operation. Adjusting the gain of each video output amplifier causes its associated cathode to produce proper white level current.
Although the technique disclosed immediately above works well, it is desirable to provide automatic tracking adjustments. This would eliminate the need for manual factory and/or service adjustments and continuously maintain proper cathode tracking.
One proposal for automatic tracking requires that the current of each cathode be sampled twice during the receiver's vertical interval. One sampling detects cathode current corresponding to black level and another sampling detects cathode current corresponding to white level. In response to the sampled currents, feedback signals are developed for use in driving the cathodes during the receiver's active scan time so that they track with one another. U.S. Pat. Nos. 4,070,691 and 4,012,775 are illustrative of this method of stabilizing cathode currents in a two step operation.
Although the two step type of automatic cathode adjustment may be useful, it introduces unwanted complexity into receiver design. Moreover, integration of the video output stages of the receiver necessarily becomes more difficult. Hence, automatic tracking or CRT stabilization networks have not been widely used in commercial television receivers.